The present invention relates to polycrystalline silicon and to a method for production thereof.
Polycrystalline silicon, abbreviated to polysilicon, is nowadays industrially produced in large amounts and is used inter alia as a raw material for applications in photovoltaics and for the production of single crystals by wafer manufacturers. In all applications, high purity of the raw material is desired.
In the production of polysilicon, after the vapor deposition it is necessary to break the polysilicon rods into small pieces for further processing. During this, however, the highly pure silicon is contaminated to a greater or lesser extent with extraneous atoms owing to the use of breaking tools. Furthermore, silicon dust particles are formed and adhere to the fragments.
Conventionally, the silicon fragments are purified for higher-quality application fields, for example pulling single crystals, before further processing and/or packaging. This is done according to the prior art in one or more wet chemical cleaning steps. Mixtures of various chemicals and/or acids are used for this, particularly in order to remove adhering extraneous atoms from the surface. These methods, however, are laborious and expensive.
It is known from U.S. Pat. No. 6,916,657 that extraneous particles or extraneous atoms can reduce the yield when pulling crystals. The adhering silicon dust can also have a detrimental effect in this regard.
A method for the determination of silicon dust on polysilicon is known from the journal “Reinraumtechnik” [Cleanroom technology] (January 2006, “Hochreiner Silziumstaub als Kontaminationsquelle” [Highly pure silicon dust as a contamination source]; Reinraumtechnik January 2006, Ivo Cröβmann). This mentions its detrimental effect during further processing to form single crystals. It is disclosed that wet-chemically cleaned polysilicon has “dust” values of about 10 ppmw and after transport up to 60 ppmw with a dust particle size distribution of less than 5 μm.
A method for preparing surface-contaminated silicon material present in a material mixture and having surface contamination of from 1 ppb to 1000 ppm expressed in terms of the weight of silicon, by screening the material adhering to the surface, is known from WO-2009/09003688. The screening does not, however, lead to a reduction of adhering Si dust smaller than about 50 μm, but rather predominantly only separation of loose and sizeable particles.
A method for preparing silicon fragments, in which a dust removal system is intended to free the silicon pieces from dust by means of an air flow through a perforated plate, is known from US 2003/0159647 A1. Low dust values are reported, but without entering into details.
Dust removal by an air flow is not however very efficient, in particular for small Si particles (<50 μm) adhering strongly to the surface. Small Si fragments can escape from the airflow. In the worst case, the strong movement of the polysilicon fragments in the fluidized bed can even lead to an increased number of particles.
It was an object of the present invention to provide economical polycrystalline silicon having a low surface dust content.